Conjoined twins are a rare complication of monochorionic twinning. Although most conjoined twins are stillborn or die in the newborn period, surgical advances are such that many parents will continue a pregnancy in the hopes that live born infants can be separated. Well-described types of conjoined twins account for most cases, with some rarer forms occasionally seen. Each type is associated with specific degrees of organ sharing, structural abnormalities, complications, and challenges with regard to potential separation. Thorough multidisciplinary prenatal assessment of conjoined twins is essential to appropriately counsel parents, to manage the pregnancy, and to create an appropriate delivery plan. This pictorial essay presents the imaging findings of the more common types of conjoined twins. Selected clinical and autopsy images are used to illustrate shared anatomy. The nomenclature for description of conjoined twins is reviewed, the imaging findings of each type are illustrated, and the challenges to separation are discussed.

Breast density assessment is an important component of the screening mammography report and conveys information to referring clinicians about mammographic sensitivity and the relative risk for developing breast cancer. These topics have gained substantial attention because of recent legislation in several states that requires patients to be informed of dense breast tissue and the potential for associated breast cancer risk and decreased mammographic sensitivity. Because of the considerable implications of diagnosing a woman with dense breast tissue, radiologists should strive to be as consistent as possible when assessing breast density. Commonly used methods of breast density assessment range from subjective visual estimation to quantitative calculations of area and volume density percentages made with complex computer algorithms. The basic principles of currently available commercial methods of calculating fibroglandular density are described and illustrated. There is no criterion standard for determining breast density, but understanding the pros and cons of the various assessment methods will allow radiologists to make informed decisions. Radiologists should understand the basic factors involved in breast density assessment, the changes related to density assessment described in the fifth edition of the American College of Radiology Breast Imaging Reporting and Data System (BI-RADS) lexicon, and the capabilities of currently available software. Online supplemental material is available for this article.

Epidemiologic/Diagnostic study, level III.